Recently, a sheet processing apparatus is arranged at a place near and following an image forming apparatus (for example, MFP) in order to finish sheets after image formation. The sheet processing apparatus is called finisher and staples or sorts and then discharges sheets send from the MFP.
The finisher has a lateral alignment unit which controls the position of sheets in a direction of the width. The finisher aligns sheets in the direction of the width and then staples the sheets with a stapler. The lateral alignment unit is also used to sort and discharge sheets. Sheets may also be stapled by manual operation. When manually stapling sheets, a sheet bundle is inserted toward the stapler from a sheet discharge port of the finisher and the stapler is then operated. However, in manual stapling, the sheet bundle cannot be correctly stapled in some cases because the sheet bundle is not aligned or the stapling position is shifted.